Equine infectious anemia virus (EIAV) is a macrophage-tropic lentivirus of horses. EIAV is unique among lentiviruses in that a further cleavage event occurs within the N-terminus of the cytoplasmic tail (CT) of transmembrane (TM) glycoprotein and yields a C-terminal non-glycosylated p20 protein. The p20 comprises more than two-third of the CT domain and contains both of the amphipathic &#945;-helices.
To test the role of the EIAV CT domain in acute disease induction, we constructed a p20-truncated clone (p19/wenv17&#916;20) on the background of a highly virulent EIAV infectious clone p19/wenv17 by introducing three termination codons into the N-terminal coding region of p20. The derived virus replicated at a delayed and lower level compared with that of parental virus in equine macrophages in vitro. In vivo, the p19/wenv17&#916;20 virus showed attenuation and did not induce acute disease like the parental (p19/wenv17) virus. The viral load in ponies infected by p19/wenv17&#916;20 virus was about 10-1000 fold lower than that of ponies infected by parental (p19/wenv17) virus. In vitro studies on the properties of the p20-truncated virus showed that truncation of the p20 did not impair the envelope glycoprotein incorporation into virions. There was also no severe defect in virus replication. The delayed and lower level replication of p20-truncated virus compared with parental virus was most probably due to small delays in several steps in the virus life cycle. In addition, p20 expressed in trans could not compensate for the absence of p20 in the p20-truncated virus.